Description:
"But of those unhappy ones who were ensnared by Melkor little is known of a certainty... Yet this is held true by the wise of Eressëa, that all those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in eny and mockery of the Elves, of whom they were afterwards the bitterest foes. For the Orcs had life and multiplied after the manner of the Children of Ilúvatar... And deep in their dark hearts the Orcs loathed the Master whom they served in fear, the maker only of their misery..."

The Silmarillion, Quenta Silmarillion, III - Of the Coming of the Elves and the Captivity of Melkor

Glamhoth = hosts of hate, the 'yelling-hoard'.Yrch is (plural) in Noldor or Sindarin for orcs; singular orch.

Morgoth's ring (HoMe 10, Part Five, Myths Transformed) contains some post-LotR speculations concerning the origins of the orcs. In addition to corrupted Elves, other possibilities mentioned are: fallen Maiar; beasts of humanized shape reeling off 'records' set by Melkor rather than talking independently; corrupted Men; and finally, mixed origins. Towards the end of his life Tolkien seems to have leaned towards the Men-hypothesis, but the question was never entirely resolved.

Etymology
In his guide for translators, "Nomenclature of The Lord of the Rings," Tolkien wrote, "I originally took the word from Old English orc (Beowulf [line] 112 orc-neas) and the gloss orc = þyrs ('ogre'), heldeofol ('hell-devil'). This is supposed not to be connected with modern English orc, ork, a name applied to various sea-beasts of the dolphin order."